This article is published with open access at Springerlink.com Abstract We propose a self-assembling procedure for the fabrication of GaAs islands by Droplet Epitaxy on silicon substrate
Trang 1S P E C I A L I S S U E A R T I C L E
Self-Assembled Local Artificial Substrates
of GaAs on Si Substrate
S Bietti•C Somaschini• N Koguchi•
C Frigeri•S Sanguinetti
Received: 21 June 2010 / Accepted: 14 August 2010 / Published online: 31 August 2010
Ó The Author(s) 2010 This article is published with open access at Springerlink.com
Abstract We propose a self-assembling procedure for the
fabrication of GaAs islands by Droplet Epitaxy on silicon
substrate Controlling substrate temperature and amount of
supplied gallium is possible to tune the base size of the
islands from 70 up to 250 nm and the density from 107to
109cm-2 The islands show a standard deviation of base
size distribution below 10% and their shape evolves
changing the aspect ratio from 0.3 to 0.5 as size increases
Due to their characteristics, these islands are suitable to be
used as local artificial substrates for the integration of III–V
quantum nanostructures directly on silicon substrate
Keywords Nanotechnology Molecular beam epitaxy
Droplet epitaxy Integration of III–V on Si Local artificial
substrate
Integration of III–V semiconductor devices on silicon
substrates would allow the fabrication of high efficiency
optoelectronic and photonic devices using the
well-estab-lished complementary metal oxide semiconductor (CMOS)
technology Although of the great technological interest,
results in this field are far from being optimized Some
issues like lattice mismatch between GaAs and Si and
formation of anti-phase domain in GaAs are not yet fully
solved [1, 2] One attractive approach to minimize these
issues is the fabrication of local artificial substrates (LAS)
The concept of LAS lies in the possibility to prepare a suitable surface for a subsequent epitaxial growth not on the whole surface of the sample, but locally, using as base some structures fabricated, for example, by a nano-pat-terning technique [3] We propose a self-assembling growth procedure based on droplet epitaxy [4,5,6] for the fabrication of GaAs islands on Si substrate with high uni-formity, selectable size, and density These islands should act as LAS for the integration of III–V high quality nanostructures on Si [7,8]
In our experiments, we used Si(001) substrates cleaned with standard RCA treatment and finally dipped into HF solution to get an H-terminated surface Substrates were loaded in a conventional GEN II molecular beam epitaxy (MBE) system equipped with an Arsenic valved cell Substrate temperature was raised to 780°C to desorb hydrogen, as confirmed with the change in surface recon-struction observed with reflection high energy electron diffraction (RHEED) from (1 9 1) to a mixed (2 9 1) (1 9 2) [9]
The substrate temperature was decreased to the value reported in Table1for the Ga deposition A Ga molecular beam flux was supplied with a deposition rate of 0.075 ML/s RHEED pattern turned to halo, suggesting liquid Ga droplets formation on Si surface The total amount of deposited Ga on each sample is indicated in Table1 Ga droplets were finally crystallized at 150°C by exposure to an As flux of 5 9 10-5
Torr for 5 min During the arsenic irradiation, the RHEED Nanoscale Res Lett (2010) 5:1905–1907
DOI 10.1007/s11671-010-9760-5
Trang 2the surface of sample A The value of island density q
reported in Table1 evidences a dependence on the
sub-strate temperature during Ga deposition It is thus possible
to use these values to finely tune island density through a
proper choice of the Ga deposition temperature Varying
the amount of deposited Ga thus makes possible to control
the mean island volume [8]
By means of AFM measurements, it is also possible to
calculate the base diameter, the height, and the size
dis-persion of the islands Table1reports the mean diameters
for the islands of each sample AFM measurements also
make possible to calculate the aspect ratio (ratio between
height and base diameter) This value increases with island
size and reaches 0.5 for the larger islands, i.e when height and radius have the same value The calculated standard deviation on base size distribution is less than 10% for all the samples This value is similar to the best values reported for SiGe islands grown on Si patterned substrate [10]
Transmission electron microscope measurements con-firmed the values obtained by AFM for the island mean base size Diffraction pattern obtained from TEM mea-surements allows also to estimate the crystalline quality of the GaAs islands Images from a single island (right panel
in Fig.2 for sample A) show two different clear Bragg spots for Si and GaAs, demonstrating the formation of single crystal GaAs islands The two small spots are due to double diffraction from GaAs and Si Diffraction images obtained with diffraction vector [220] (left panel in Fig.2
for sample E) evidence Moire´ fringes The spacing between fringes gives a good estimation for distances between planes [220] in Si and GaAs, and the values we obtained are in agreement with the ones expected for bulk material, thus demonstrating fully relaxation of GaAs islands Figure3 reports the typical profile of randomly chosen islands on samples B (left panels), D (central panels,
Table 1 Ga deposition temperature, Ga coverage, average diameter
and density of the islands
Sample Tsub(°C) MLs d (nm) q (islands/lm-2)
Fig 1 Large area TEM scans
on surface of sample E (left
panel) and A (right panel)
Fig 2 Left panel: TEM image
of a single island on sample E
obtained with diffraction vector
[220] Right panel: (220) Bragg
spot on diffraction pattern from
a single island on sample A
Trang 3magnified by a factor of 2), and E (right panels magnified
by a factor of 3) along [110] (upper panels) and [1–10]
(lower panels) Profiles are obtained from high-resolution
AFM images The slope exposed by the islands is mostly
compatible with the plane {111} near the Si surface and
with the plane {113} in the upper part of the islands A
similar behavior with the presence of these planes was
observed also in Volmer-Weber growth of GaAs on Si
[11] However, by using the droplet, epitaxy technique is
possible to achieve a clear improvement in size
distribu-tion From the profiles obtained, we could correlate the
increment in the aspect ratio from 0.3 up to 0.5 with the
exposed planes on the profiles The bigger islands expose
more portions of surface with {111} slope and reduce
{113} The formation of different surface planes plays an
important role for the subsequent fabrication of small
quantum dots on the top of local substrates A stronger
affinity for the nucleation of InAs small quantum dots on
high index surface like {113} or {115} than on {111} is
reported [7] We expect that the control on exposed
sur-faces could allow the selection of a particular area of the
LAS as a preferential nucleation site for InAs quantum
dots
We have presented a fabrication procedure for the
self-assembly of GaAs islands on Si substrate via droplet
epi-taxy to be used as LAS The islands are made by a single
relaxed crystal and are tunable in size and density changing
substrate temperature and amount of supplied Ga The
islands expose large portions of surface with a slope
compatible with {111} and {113}, and the aspect ratio
reaches 0.5 for larger islands The control on size, density,
and exposed surfaces, the well-defined shape and the low
thermal budget requirements make these islands good candidates for the fabrication of LAS and the integration of III–V nanostructures on Si substrate
Acknowledgments This work was supported by the CARIPLO Foundation under the project QUADIS2 (Contract no 2008-3186) and by the Italian PRIN-MIUR under the project GOCCIA (Contract
no 2008CH5N34).
Open Access This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which per-mits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
References
1 Y.B Bolkhovityanov, O.P Pchelyakov, Phys Usp 51, 437–456 (2008)
2 S Bietti, C Somaschini, S Sanguinetti, N Koguchi, G Isella, D Chrastina, Appl Phys Lett 95, 241102 (2009)
3 J He, K Yadavalli, Z Zhao, N Li, Z Hao, K.L Wang, A.P Jacob, Nanotechnology 19, 455607 (2008)
4 N Koguchi, S Takahashi, T Chikyow, J Cryst Growth 111, 688–692 (1991)
5 N Koguchi, K Ishige, Jpn J Appl Phys 32, 2052–2058 (1993)
6 C Somaschini, S Bietti, N Koguchi, S Sanguinetti, Nano Lett.
9, 3419–24 (2009)
7 P.S Wong, G Balakrishnan, N Nuntawong, J Tatebayashi, D.L Huffaker, Appl Phys Lett 90, 183103 (2007)
8 C Somaschini, S Bietti, N Koguchi, F Montalenti, C Frigeri, S Sanguinetti, Appl Phys Lett 97, 053101 (2010)
9 Z.M Zhao, O Hulko, T.S Yoon, Y.H Xie, J Appl Phys 98,
123526 (2005)
10 Z Zhong, G Bauer, Appl Phys Lett 84, 1922 (2004)
11 H Usui, H Yasuda, H Mori, Thin Solid Films 516, 2487–2490 (2008)
Fig 3 AFM profiles along
[110] (upper panels) and [1–10]
(lower panels) for islands on
samples B (two left panels), D
(two center panels) and C
(two right panels)